Have you considered a thigh holster? That's what I use for my 1911. It attaches to your belt and then your thigh, dropping the gun below the waistline (so it doesn't conflict with the waist belt of your backpack).

Chest holsters seem like they wound be uncomfortable, as the straps usually go over one or both shoulders and then come around your side, thus would be sitting underneath your pack straps. That would probably bug me a lot on long hikes.

I don't own a revolver, so I'm not sure what the options are out there for them, but for automatics Blackhawk makes a good product, plus you could probably find a generic nylon thigh holster pretty easily.

Edit: Shot my buddy a text who hikes with a revolver and he said to research the "border patrol" style revolver holster... it sits on your belt but places the gun lower so it won't interfere with your waist belt.